1. Field of the Invention
This invention relates generally to a powertrain control system algorithm that increases shift point engine speed of heavily loaded vehicles to prevent powertrain hunting, but allows upshifts.
2. Description of the Prior Art
During operation of a automatic transmission for a motor vehicle a cycle can occurs, during which the transmission upshifts automatically, vehicle speed slows, the transmission downshifts automatically, causing vehicle speed to increase. This cycle is often called powertrain hunting.
A control strategy to prevent powertrain hunting (PPH) would inhibit transmission upshifts to a higher gear, if the upshift would prevent vehicle acceleration in the upshifted gear. If the transmission upshifts with the vehicle heavily loaded or ascending a grade, vehicle speed would decrease and quickly cause a downshift.
Powertrain hunting can be avoided by preventing an upshift and holding the transmission in the lower gear. When this strategy is applied to a high speed truck powertrain, the engine can overheat due to the inability of the engine cooling fan to adequately cool the powertrain at sustained high engine speed.
Heavily loaded trucks can drive for several miles up steep grades, held at a lower gear with the engine running at high speed. This high speed operation causes powertrain overheating.